The present invention relates to pipes for conveying drinking water and more particularly polyolefin pipes. Mains for distributing drinking water, in particular buried mains are made of polyolefin pipes such as polyethylene or polypropylene. These pipes have many advantages. They do not corrode, they are delivered as coils of great length; it suffices to unroll them.
Polyethylene and polypropylene have no need of plasticizers; they are highly suitable for drinking water. However, some soils can be contaminated or risk being contaminated by essentially nonpolar hydrocarbons like toluene, benzene, fuel or motor spirit.
Since polyolefins are nonpolar materials, they are permeable to these contaminants, which migrate through the wall of the pipe and contaminate the water.
The applicants have now found new pipes for conveying drinking water, which are impervious or substantially impervious to hydrocarbons and to relatively nonpolar organic molecules containing oxygen, sulfur and/or nitrogen while retaining the advantages of the polyolefin pipes.
The present invention relates to pipes including a main layer made of polyolefin covered externally with a layer of a thermoplastic polymer which is impervious or substantially impervious to hydrocarbons and relatively nonpolar organic molecules. Hereinafter, the term xe2x80x9cimperviousxe2x80x9d shall indicate xe2x80x9cimpervious or substantially imperviousxe2x80x9d. The invention includes pipes for conveying drinking water, comprised of a plurality of layers, wherein a main layer comprising a polyolefin is covered externally with a second layer comprising a thermoplastic polymer which is impervious to hydrocarbons.
The polyolefin of the main layer may be chosen from polyethylene, polypropylene, copolymers of ethylene and of an alpha-olefin, copolymers of ethylene and of esters of unsaturated carboxylic acids, such as alkyl (meth)acrylates, copolymers of ethylene and of vinyl derivatives of saturated carboxylic acids such as vinyl acetate, or mixtures of these polymers.
Low density, high density or intermediate density polyethylene is advantageously employed. The polyethylene is usually filled with carbon black.
It would not constitute a departure from the scope of the invention if the polyolefin layer consisted of a number of layers, for example a layer of recycled polyolefin between two layers of virgin polyolefin.
The thickness of this polyolefin layer is a function of the mechanical stresses. To give an example, a thickness of 30 to 60 mm is suitable for external diameters up to 800 mm, while thicknesses of 2 to 6 mm are employed for external diameters of 20 mm (see, for example, NFT standard 54-063 for HDPE pipes).